It is known to provide infusion sets, blood collection and transfer bags and a variety of other medical units for administration of liquids to a patient or for the transfer of solutions between vessels which utilize plastic tubing. Such tubing may be polyvinylchloride tubing or other thermoplastic tubing.
Frequently it is important to hermetically seal the tubing and, for this purpose, high-frequency or radio-frequency systems have been provided heretofore.
A typical system for this purpose can include a radio-frequency generator connected to a sealing head provided with a pair of electrodes adapted to pinch the tubing between them and, when the electrodes are supplied with the radio-frequency energy, to cause fusion of the internal walls of the pinched tubing and thus a hermetically closed seal at the pinch. The sealing jaws or electrodes can function as capacitor plates in a tuned network or circuit supplied with a high-frequency energy, see the commonly owned copending application Ser. No. 07/820,040 filed Jan. 13, 1992.
Prior art circuitry and systems of this type are described in U.S. Pat. Nos. 4,013,860, 4,186,292, 4,390,832, 4,490,598, 4,529,832, 4,488,028, 4,491,711 and 4,496,819 assigned to Engineering and Research Associates, Inc. of Tucson, Ariz.
The systems described in these patents utilize an RF generator which generates electrical energy with a fundamental frequency of 40.68 MHz that is connected to a sealing head with a 50 ohm coaxial cable. The sealing head may transform the supplied energy to a high voltage of, said 1500 volts, as has been found to be necessary to generate sufficient heat in the tubing to cause it to melt.
One of the drawbacks of these earlier devices for sealing medical plastic tubing is that in addition to the fundamental frequency of high energy which is produced, two types of undesirable frequencies are also generated and emitted.
One of these types is a group of frequencies that are close to the fundamental frequency and these undesired frequencies may be caused by an incorrect choice or mounting of the components of the electrical circuit.
Interference from these incorrect frequencies can be prevented by the selection of correct components and by the mounting of them and the trimming of them so that the spurious radiation of the first type will not be generated.
The second type of spurious frequencies are multiples of the fundamental frequency. These undesirable frequencies are a result of the action of nonlinear components, like electron tubes or transistors which cause nonlinear distortions.
The original sinusoid of the signal is distorted to a non-sinusoid signal. Such a non-sinusoid can be seen as the sum of the original frequency and one or more multiples thereof (Fourier-analysis). As these multiples of the fundamental frequency, the so-called harmonics, are amplified by each amplifier stage, they will reach the sealing head. During the total cycle of generating the signal, the amplification and applying the signal to the sealing head, the harmonics will be emitted.
When an electric signal is supplied to a linear circuit, e.g. a voltage divider containing two resistors, it will undergo a certain processing.
Suppose the voltage divider weakens the signal by a factor of two, then the signal will be exactly the same coming out of the divider, only two times weaker.
When a sinusoidal signal is supplied to such a factor two resistor voltage divider, it can be formulated: EQU V.sub.OUT =V.sub.in .times.V.sub.char
where
V.sub.out =the outgoing voltage PA1 V.sub.in =the supplied sinusoid AC voltage being sin(2.pi.f) PA1 V.sub.char =the transfer characteristic of the circuit, being 0.5 in this case. PA1 V.sub.out =the outgoing voltage PA1 V.sub.in =the supplied voltage, being x+sin(2.pi.f) PA1 f=resonance frequency in Hertz PA1 L=self-induction in Henry PA1 C=capacity in Farad
making EQU V.sub.out =sin(2.pi.f).times.0.5=1/2 sin(2.pi.f).
As can be seen, the frequency of the signal will stay untouched when the resistors are linear components with a constant transfer characteristic, independent of the level of the voltage. No signals will be generated on undesired frequencies.
However as the concept of a RF generator is based on signal amplifying, it is necessary to make use of transistors and electron tubes. These components have a non-linear transfer characteristic. This means that the transfer of the signal is not constant but depending on the voltage of this signal.
When we supply a sinusoid signal to a non-linear component, this signal will be deformed due to the curvature of the transfer characteristic. The original signal will therefore change in magnitude and in shape.
The Fourier analysis indicates that every form of signal can be decomposed into sinusoid signals, including a fundamental frequency and one or more spurious frequencies.
Suppose a sinusoidal signal is supplied to a component with a pure square characteristic, which can be formulated: EQU V.sub.out =(V.sub.in).sup.2
where
making EQU V.sub.out ={x+sin(2.pi.f)}.sup.2 =x.sup.2 +2x sin(2.pi.f)+1/2-1/2 cos(4.pi.f)
It can be seen (using the half angle relationship cos 2.alpha.=1-2 sin.sup.2 .alpha.) that a frequency is formed that is twice the fundamental frequency. This is called the second harmonic. However non-linear components, as this amplifier component, seldom have a pure square character, but will mostly have a much more complicated form. This means that not only a second harmonic will be formed, but a large number of harmonics, all having in common that they are multiples of the fundamental frequency.
It can be concluded that due to the application of amplifying components as transistors and electron tubes in a RF generator, spurious radiation will be generated on frequencies that lie outside the fundamental frequency.
The generation of RF energy is limited by national and international standards concerning the frequencies that are open for emitting and the amount of spurious radiation that is allowed on other frequencies. This is e.g. established in the international standards such as the CENELEC, EN 55022, CISPR 11 and the VDE 0871. In Germany the devices as described in this invention are subject to the standards of VDE 0871.
In most cases the harmonics of the fundamental frequency lie in areas of frequencies where strong limits apply on the amount of radiation that is emitted. When the devices that are on the market at the moment are tested according to the VDE standards, it is found that they are far from those standards.
The consequences of being off standard of these devices are very noticeable in is bloodbanks and laboratories. Not only the radio reception heavily disturbed, but also the emitting of spurious radiation can cause fine tuned laboratory machinery to fail, making test results insecure. In general this kind of radiation can cause all sorts of undesired radio interference in telecommunications for aeronautics, telephone, etc.